Founding Member

Opinions please! My stock 289 2v has ported vac advance. I've heard manifold vacuum can improve mileage. True or not? I'm also trying to get it to run better on this lite beer that passes for gas in Arizona these days. I've backed the vac advance diaphragm off to almost nothing and still get a little ping on acceleration. I'm thinking the manifold connection would help that.

Founding Member

With my stock 2 barrel 302 I ran it to manifold vacuum. I'm not really sure if it helps mileage. When I went with the 4 barrel 351W I ran it to ported. I tried the manifold vacuum, but it stumbled and didn't idle as well. So it's all in what your combo likes.

If you put it to manifold vacuum you will have maximun advance at idle and no advance under wide open throttle....exactly the opposite of how it is supposed to work. Regardless of what anybody tells you the vacuum advance is ALWAYS hooked to porter (venturi) vacuum.

Founding Member

With my stock 2 barrel 302 I ran it to manifold vacuum. I'm not really sure if it helps mileage. When I went with the 4 barrel 351W I ran it to ported. I tried the manifold vacuum, but it stumbled and didn't idle as well. So it's all in what your combo likes.

In response to your question...yes, the 351 had a hotter cam then the 302.

As for always going to ported vacuum, didn't the cars come from the factory hooked up to manifold vacuum?

Click to expand...

Nope...it is always hooked to ported vacuum from the factory as it is the only way for it to be hooked up properly. Some people think manifold vacuum is ok because their car pings on ported but does not when hooked to manifold....the reason is they are now using their vacuum advance as a vacuum retard so to speak and need to recurve the distributor or back off initial timing to get the proper overall advance curve.

Founding Member

Some people think manifold vacuum is ok because their car pings on ported but does not when hooked to manifold....the reason is they are now using their vacuum advance as a vacuum retard so to speak and need to recurve the distributor or back off initial timing to get the proper overall advance curve.

You never run the vacuum advance to a manifold vacuum source regardless of engine size, camhaft size, or any other factor. It ALWAYS goes to ported vacuum....ALWAYS.

Click to expand...

You are sooooooooooooooooo right on here. I can't even believe we are talking about ANYTHING else. I guess if a guy wanted to run off the manifold, he could severly retard the timing, but it would defeat the porpose. Besides, how would it start anyway?

Founding Member

If you put it to manifold vacuum you will have maximun advance at idle and no advance under wide open throttle....exactly the opposite of how it is supposed to work. Regardless of what anybody tells you the vacuum advance is ALWAYS hooked to porter (venturi) vacuum.

My thinking is, you have max vac advance at idle (or lo RPM) because the engine turns too slowly to allow the mechanical adv to work (the lil springs/weights)....engine wont run right at idle without it....vac is 'assisting' the advance....

As RPMs increase, manifold vac/venturi vac decrease...lessening the effect of the vac advance...but the increased centrifugal starts the mech adv up the 'curve'....depending on the curve, the vac adv cuts out entirely and its mech from there (to a max value ~ 3500 or so, usually ~ 36-38* for small block Ford).

So how can its be 'opposite?" Hi manifold vac at high RPM and low manifold vac at idle/lo RPM? Nuh uh.

My thinking is, you have max vac advance at idle (or lo RPM) because the engine turns too slowly to allow the mechanical adv to work (the lil springs/weights)....engine wont run right at idle without it....vac is 'assisting' the advance....

As RPMs increase, manifold vac/venturi vac decrease...lessening the effect of the vac advance...but the increased centrifugal starts the mech adv up the 'curve'....depending on the curve, the vac adv cuts out entirely and its mech from there (to a max value ~ 3500 or so, usually ~ 36-38* for small block Ford).

So how can its be 'opposite?" Hi manifold vac at high RPM and low manifold vac at idle/lo RPM? Nuh uh.

Edumucate me

Click to expand...

Simple. If it sucks more plugged into the manifold than when plugged into a carb port, the vac advance will be higher. Mechanical advance doesn't come into play. Besides, some folks don't have mech advance.

Actually, vacuum advance is pretty high at freeway cruising, as is vacuum. When you mash the throttle, timing retards a little cuz vacuum drops. Prevents detonation during higher cylinder pressures of WOT driving. Think of vacuum advance as fine economy tuning for mechanical advance. Vacuum allows more advance during light throttle cruising, which improves economy. When you mash it, it goes away. In the days of just mechanical, it would just run a little more retarded all the time.

It's a fairly fine tuned system, and anytime you make any engine mods, you alter it's effectiveness. That's why so many people with heavily modified engines just go to straight mechanical. Economy is less of a concern and it's hard to make mechanical and vacuum advance work perfectly together on your own.

Sure, manifold vac decreases, but it is the AMOUNT of vacumm that comes into play. The amount of suck on the manifold is what????? 1000 times greater than ported? That is where it will grossly increase the vac advance.

Founding Member

Yikes! Discourse, discussion, debate hmmm. I understand that the manifold connection essentially becomes vacuum retard at wide open throttle. That's one reason I was thinking it would help. Currently I have my initial timing set by the vacuum gauge, according to the Pony Carbs reccomendation. This puts initial way past the factory specs. And the vac advance just adds to it when you mash the pedal. I was thinking the reduction in advance at the right point would help control ping.

If you put it to manifold vacuum you will have maximun advance at idle and no advance under wide open throttle....exactly the opposite of how it is supposed to work. Regardless of what anybody tells you the vacuum advance is ALWAYS hooked to porter (venturi) vacuum.

Click to expand...

As far as I know, ported and manifold vacuum should have the same amount of vacuum except then the throttle plates are closed, in which case, there will be no vacuum from the ported vacuum port. I personally tested this with a vacuum gauge on a Holley carb and the part throttle and WOT readings where pretty much the same between the ported and manifold vacuum ports.